The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates generally to microwave resonators and more particularly to N-section microwave resonators with variable coupling.
2. Description of the Prior Art
In microwave resonators and filters with exact performance requirements, it is desirable to employ tuning adjustments to alleviate the need for unreasonable fabrication tolerances. In rectangular waveguide filters, the coupling between filter cavities is made adjustable by the use of a tuning screw in the plane of the coupling iris. However, in coupled high Q, cylindrical cavity resonators, the iris plates are relatively thin in comparison to the diameter of a tuning screw, which makes the use of such screws impractical.
When operating a resonator in the TE.sub.11 circular mode, the coupling between resonators is provided by apertures in the iris plates, and the aperture size varies along the length of the resonator. The size of the aperture depends upon the reactance necessary to provide the required rf bandwidth, while the reactance is related to the ratio of the thickness, t, of the iris wall and d, the aperture opening. It has been found that because of manufacturing tolerances of both d and t, some variable means is needed to provide the required electrical performance of the components at microwave frequencies.
Generally, for accuracy and convenience, the aperture openings have been in the form of a circular hole centered in the iris plate. For the first approximation, if t remains constant, then the reactance or coupling between cavities depends upon the width of the hole with respect to the propagating TE.sub.11 mode. In a resonant cavity of .lambda.g/2 in length, current density is high at the aperture location, while a minimum occurs at .lambda.g/4 from the aperture location. This is at the midpoint of the resonator.
Even where the iris plates are thick enough to accommodate tuning screws in order to provide variable coupling, the insertion of the screw into the aperture increases coupling, but at the expense of increasing the insertion loss of the final resonator assembly.
In contradistinction to the prior art, in the present invention the resonator is constructed with a rotary joint between the iris plates at the midpoint of the resonator, and the apertures in the plates are in the form of ellipses whereby the coupling into and out of the resonator may be accurately varied simply by rotating the portions of the waveguide on opposite sides of the rotating joint with respect to each other. In this manner, both over and under coupling values are obtained, the range only depending upon the dimensions of the major and minor axes of the ellipses.
In the case of an N-section resonator, normally, the coupling aperture opening is large for the end resonator and smallest at the midsection. Therefore, if the major and minor ellipse openings are selected from the maximum and minimum aperture dimensions required for a given N-section resonator to provide a given electrical characteristic, all that is required in the invention is to provide identical elliptical shape apertures for all sections and be able to rotate the portions of each section on opposite sides of the rotating joint 90.degree. to obtain infinitely variable coupling values as determined by the maximum and minimum aperture sizes.